# Robert Ellis

## In Memoriam

Robert Mortimer Ellis was born in Cleveland, Ohio on September 16, 1926. He died on December 6, 2013.

Bob grew up in Philadelphia, and after a brief stint in the army studied in the Moore School of Engineering at the University of Pennsylvania. It was while he was there that he learned that his calling was mathematics. He received his Ph.D. at Penn in 1953, under Walter Gottschalk, and went on to a post-doctoral position at the University of Chicago where he met Elizabeth Wood, his future wife, who was then studying at the Divinity School there. They married and remained partners for the next fifty years.

After Chicago, Bob returned to the east coast and first taught at Pennsylvania State University between 1955 and 1963 where he was appointed Assistant Professor, and then in 1957 Associate Professor. He left Penn State to become a Professor at Wesleyan University in 1963. In 1967 he moved to Minnesota where he became a Professor in our department until his retirement in 1995.

Bob's life-long research interest was in the area of Topological Dynamics. In pioneering work from the 1950s he laid the foundations of an algebraic approach to this area, shaping many instruments that proved to be powerful tools. A particular example is the notion of an enveloping semigroup', which is now known as theEllis semigroup', of a dynamical system. With such algebraic machinery he developed a deeper understanding of several fundamental relations in Topological Dynamics, such as the equicontinuity structure relation and the distal structure relation. This study resulted in what is now known as Ellis's Galois theory of distal extensions. The work leads to an alternate approach to proving deep structure theorems such as Furstenberg's structure theorem.

Altogether Bob was the author of about 40 research publications, which include a highly cited book from 1969 on topological dynamics in which he gave an exposition of his algebraic theory. The most recent publication is a book written with his son David entitled, 'Automorphisms and equivalence relations in topological dynamics', to be published by Cambridge University Press this year. David is Professor of Mathematics at Beloit College and has been a long-time collaborator. At the time of his retirement a conference was held in Bob’s honor, on April 5-6 1995, at the University of Minnesota. The proceedings of this conference appeared as volume 215 in the AMS series Contemporary Mathematics.

In addition to his research, Bob was a dedicated teacher. He was the advisor of five Ph.D. students, one at Wesleyan and four at Minnesota and he is remembered fondly by these, as well as by undergraduates and graduate students who took his courses. Although he was dedicated to his research, he always believed in the importance of education and frequently gained solace in it when he felt his own work was not going well.

Bob lived in Minnesota for several years after retirement until he moved to Madison WI, where he continued to be active in research in retirement. He was elected a Fellow of the American Mathematical Society in the inaugural list of Fellows. He was involved in the care of his wife Betty until her death. He is survived by his son, a daughter and one grandchild.